Low-resolution video coding content extraction

ABSTRACT

Low complexity method embodiments directly decode low-resolution frames from compressed high-resolution videos that were encoded using predictive coding techniques like the H.264 video coding standard. The smaller the decoding resolution, the higher will be the computation and power savings of using the method. Low-frequency coefficients of 2D transformed predictions are added to the low-frequency coefficients of the transformed residual error. Low-frequency coefficients of the reconstructed data are then inverse transformed taking a smaller size transform. Further savings are obtained by reconstructing only those reference pixels that will be needed for accurate decoding of further Intra blocks.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of application Ser. No. 12/346,936,filed Dec. 31, 2008, which is incorporated herein by reference in itsentirety.

The embodiments relate generally to methods and devices for efficientlyextracting video thumbnails and low-resolution pictures from compressedhigh-resolution bitstreams coded using H.264 and other video codingstandards which are based on predictive compression techniques, and moreparticularly to the adding of low-frequency coefficients of 2Dtransformed predictions to the low-frequency coefficients of thetransformed residual error, and the low-frequency coefficients of thereconstructed data are then inverse transformed taking a smaller size,and reconstructing only those reference pixels that will be needed foraccurate decoding of further Intra blocks.

Given the same video quality results, the Video Coding Standard H.264can compress video files to half the file size as previous video codingstandards like MPEG4. The degree of compression depends on how close thepredicted data is to the original data to be coded. But PredictiveCoding has an overhead because it requires the inclusion of referencedata that is needed later to generate the predicted data.

Both the Inter and Intra-frame compression techniques used in VideoCoding Standard H.264 are based on predictive coding. It is thereforerelatively more complex when compared with video coding standards suchas MPEG2, MPEG4, VC-1, etc. Obtaining even low-resolution videoframes/video thumbnails from H.264 video files can be very complex, atleast when using conventional methods. Nevertheless, being able todecode lower resolution frames from high-resolution compressed videoframes is desirable for a lot of reasons.

Conventional ways to decode lower resolution frames from high-resolutioncompressed video frames include full frame decoding and downscaling,partial frame decoding, and decoding from a hierarchically codedbit-stream. A full resolution image is decoded and then the fullresolution decoded image is scaled down to the desired lower resolution.Such scaling usually includes anti-aliasing filtering/averaging and downsampling.

In partial frame decoding, the data in many bitstreams is available inthe transform domain, e.g., JPEG, Intra Frame of Video Coding standardssuch as WMV7, WMV8, WMV9, MPEG1, MPEG2, MPEG4 and 11261, H.263 etc. Itis therefore possible to decode low-resolution frames by simply decodinga few, low-frequency coefficients. MPEG4 uses AC and DC prediction inthe transformed domain, so the AC and DC prediction is done prior to thedecoding of a low-resolution frame. Instead of taking an 8×8 inversetransform, a 1'1, 2×2, or 4×4 inverse transform is taken of the 1×1,2×2, or 4'4 block located in a larger block, like an 8×8 block.

In hierarchically coded bitstreams, the bitstreams are encoded with botha low-resolution bitstream and a corresponding enhancement layerbitstream. Just the low-resolution bitstreams need to be decoded to getlow-resolution images or video. Getting the high-resolution image/videoframes includes decoding both the low-resolution and high-resolutionbitstreams.

H.264 encodes Intra information differently than do previous videocoding standards like MPEG1, MPEG2, MPEG4, H.263, WMV7, WMV8, etc. Aprediction for a current block is generated from reference pixels thatare in the top and left side of the current block. These referencepixels are already encoded and decoded, and are available for generatingthe prediction for the current block. The prediction generated is thensubtracted from the current block, and a residual error is obtained,e.g., Residual Block=Current Block−Prediction Block. The residual blockis transformed, quantized, and the run length symbols generated areentropy coded. The coded residual block and the coded prediction modeare then formatted into a video bitstream.

H.264 uses various block sizes and various prediction modes for coding.H.264 currently uses 16×16, 8×8 and 4×4 block sizes to code the dataaccording to the Intra compression method.

In the coding of the luminance 16×16 Intra prediction mode according toH.264, the data for the current Intra Luminance 16×16 Block is predictedin four ways:

-   -   Intra 16×16 luminance Mode 0—Prediction in the Vertical        direction;    -   Intra 16×16 luminance Mode 1—Prediction in the Horizontal        direction;    -   Intra 16×16 luminance Mode 2—DC Prediction; and    -   Intra 16×16 luminance Mode 3—Plane Prediction. Reference Pixels        at the top and left side are used to code a 16×16 block.

In the coding of Chrominance 8×8 Intra Prediction Mode according toH.264, the data for the current Intra Chrominance 8×8 Block is predictedin four ways:

-   -   Intra 8×8 chrominance Mode 0—DC Prediction Mode;    -   Intra 8×8 chrominance Mode 1—Horizontal Prediction Mode;    -   Intra 8×8 chrominance Mode 2—Vertical Prediction Mode; and    -   Intra 8×8 chrominance Mode 3—Plane Prediction.

For the encoding of Luminance Intra 4×4 Blocks, Luminance Intra 4×4Prediction Mode prediction is generated from the pixels (I to L, M, andA to H) that lie to the immediate left and top of a current block.

TABLE-I

In Table-I, the sixteen pixels labeled “a” to “ p” represent a current4×4 block to be coded. Pixels M, and A-H are neighboring referencepixels immediately to the left and above that are used in nine differentways to generate a prediction for the current block along the verticaldirection, the horizontal direction, DC, the diagonal down leftdirection, diagonal down right direction, the vertical right direction,the horizontal down direction, the vertical left direction, and thehorizontal up direction.

H.264 uses predictive coding to code the Intra prediction mode of thecurrent Intra block. It uses a flag to indicate whether the predictedmode is to be used or not. If a predicted mode is not used, it sendsthree extra bits to specify the current prediction mode.

TABLE II

In an example represented in Table-II, a block C is a current block tobe coded given neighboring blocks A and B. A prediction,“predIntraC×CPredMode” for the Intra prediction mode of current Intrablock is generated in the following way:

predIntraCxCPredMode = min (intraMxMPredModeA, intraMxMPredModeB), where A and B can be of the same block size as C, or A and B can have ablock size larger than C. For example, A can be of size 4x4 and B can beof size 8x8. If (predIntraCxCPredMode = = Intra Prediction Mode ofcurrent Block)  Use_Pred_Mode_Flag = 1 Else  Use_Pred_Mode_Flag = 0.When, Use_Pred_Mode_Flag is zero, three bits follow it to specify one ofeight remaining prediction modes, if (CurrentIntraMode <predIntraCxCPredMode)  RemIntraMode = CurrentIntraMode Else RemIntraMode = CurrentIntraMode −1

A typical Set Top Box is an application in which multiple channels areavailable for decode and display. What is needed is a quick, low powermethod to allow a user to see snapshots of multiple video bitstreams sothey can choose that video to play. But, conventional decoding anddisplay of multiple H.264 video streams would ordinarily be a time andpower consuming task.

In an embodiment, low-resolution video frames/video thumbnails aredecoded from compressed high-resolution videos that were encoded usingInfra predictive coding techniques like the H.264 video coding standard.Smaller decoding resolutions produce greater computation and powersavings using the method. Low resolution frames of non-reference B and Pframes are directly reconstructed. An example of non-reference B/P frameis a B/P frame present immediately before an DR frame. The low-frequencycoefficients of 2D transformed predictions are added to thelow-frequency coefficients of the transformed residual error.Low-frequency coefficients of the reconstructed data are then inversetransformed taking a smaller size. Further savings are obtained byreconstructing only those reference pixels that will be needed fordecoding further Intra blocks.

A system, device, protocol, and method are described. Other embodimentsof the system and method are also described.

Other aspects and advantages of the present embodiments will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the embodiments.

FIG. 1 shows how a video coding standard H.264 compressed video inputcomprises sequences of I-frames, B-frames, P-frames, and IDR-frames. Andhow the last P-frame or B-frame just before the IDR-frame is the framewhose low resolution frame is directly reconstructed as a video frame ofvideo thumbnail in the embodiments.

FIG. 2 shows a method embodiment for extracting low resolution videoframes/video frames of video thumbnails from H.264 video bitstreams.

FIG. 3 shows a decoder for extracting low resolution video frames/videoframes of video thumbnails from H.264 video bitstreams.

FIG. 4 represents a set top box embodiment in which several videothumbnails are displayed to a user to help with program selection.

FIG. 5 represents a method embodiment for sub-sampling of Intra blocks.

FIG. 6 represents a method for sub-sampling of Inter blocks.

FIG. 7 represents a video thumbnail method embodiment.

Throughout the description, similar reference numbers may be used toidentify similar elements.

In the following description, specific details of various embodimentsare provided. However, some embodiments may be practiced with less thanall of these specific details. In other instances, certain methods,procedures, components, structures, and/or functions are described in nomore detail than is necessary to enable the various embodiments of theembodiments, e.g., for the sake of brevity and clarity.

In general, the embodiments allow users to see a snapshots of multiplevideo bit-streams so they can choose which video to play. In a typicalSet Top Box application, there are multiple channels to simultaneouslydecode and display to a user trying to make a program selection. Thedecoding and display of multiple video streams that use predictivecoding can be computationally tedious and power consuming, even for theextraction of low-resolution frames. This is because the high-resolutionimages ordinarily have to be fully decoded before they can be scaleddown to lower resolution. The embodiments decode low-resolution framesin relatively less time and with less power consumption, by simplifyingsome of the decoding steps taken and reducing the transform sizes.

As represented in FIG. 1, an H.264 compressed video bitstream 100 inputto a decoder embodiment consists of groups of pictures (GOP) that beginwith an Intra-compressed frame (I-frame) followed by a number ofInter-predicted frames (P-frames) from previous I or P frames, andbi-directional predicted frames (B-frames). The I-frames are compressedwithout reference to other frames. The B-frames require less data thanP-frames. The P-frames are predicted based on prior I-frames or P-framesplus the addition of data for changed macroblocks. All the frames neededfor the predictions are included in each corresponding GOP.

An Instantaneous Decoding Refresh (IDR) I-frame 102 marks a closed H.264group of pictures (GOP) by allowing subsequent frames to referencethemselves and the frames after it. This resets the decoder completelyand guarantees reliable seeking. In H.264, any frames following an IDRframe may not refer back to frames preceding the IDR frame. Non IDRI-frames are, in effect, Intra-coded P-frames. They can be referenced bypreceding B-frames.

In FIG. 1, a last frame 104, either a B-frame or a P-frame, before theIDR frame 102 is selected. A sub-sample 106 is taken for reconstructionat a resolution much lower than originally encoded. A low-resolutiondecoder 108 reconstructs low-resolution video frame/video frame of videothumbnails 110.

H.264 exploits spatial correlation in Intra coded macroblocks with itsIntra-prediction techniques. Inputs to this process are thereconstructed macroblocks, the Intra prediction modes of the currentmacroblock, and the previously decoded neighboring macroblocks. Theoutput is constructed samples taken prior to the de-blocking filter.H.264 normally can apply nine modes of Intra-prediction on 4×4 blocksand 8×8 blocks, and four modes on 16×16 blocks.

H.264 uses modified 4×4 integer DCT to avoid mismatches between theencoders and decoders. The scaling multiplication of transformation isintegrated into the quantization process. H.264 further exploitscorrelation in sixteen DC values of the transformed macroblock byapplying a 4×4 Hadamard transform on luminance DC coefficients and a 2×2Hadamard transform on chrominance DC components. An entropy decoded andinverse zigzag scanned macroblocks output will be inverse quantized andinverse transformed to produce reconstructed macroblocks.

To reduce the blocking artifacts introduced by block-based transforms,inter-predictions, Intra-predictions, and quantization, H.264 includesan adaptive and optional in-loop deblocking filter. Filtering is appliedadaptively along the 4×4 block edges. The inputs to the filter arecompletely reconstructed macroblocks, boundary strength, andquantization parameters. The filter outputs are the final reconstructedmacroblocks.

FIG. 2 represents a decoder method embodiment 200 for extractinglow-resolution video frames/video frames of video thumbnails from ahigh-resolution video input 202 that was compressed with the H.264 videocoding standard. It would also work with other similar type videocompression that uses predictive coding. In method 200, Low resolutionnon-reference B and P frames are directly reconstructed. An example ofnon-reference B/P frame is a B/P frame present immediately before an IDRframe. These P-frames and B-frames do not require de-blocking, becauseat these resolutions the human eye will do the averaging and blocking,making any artifacts not visible. I Frame or Non-reference P/B framesare provided to an input 202.

A step 204 does entropy decoding according to the H.264 video codingstandard. Entropy encoding is used to store large amounts of data byexamining the frequency of patterns within it and encoding this inanother, smaller form. A step 206 does an inverse zigzag scan order toform a block of quantized transformed residuals.

Part of this output is provided to a step 208 that does either a 2×2 or4×4 inverse transform on the DC coefficients of the Intra-8×8chrominance or Intra-16×16 luminance blocks only which are coded usingIntra-8×8 chrominance or Intra-16×16 luminance Prediction modes. Thisoutput, and another part of the output of step 206 are input to a step210 of Re-scaling. Re-scaling is a combination of prescaling and inversequantization operations. Prescaling is an scaling operation which is apart of Modified Integer IDCT.

In step 210, the quantized transform coefficients are re-scaled. Eachcoefficient is multiplied by an integer value to restore its originalscale.

A step 212 adds the transformed residuals to the correspondingtransformed predictions, and outputs transformed reconstructedmacroblocks. The decoder adds the transformed prediction to thetransformed decoded residual to reconstruct a transformed decodeddownsampled macroblock which will become a part of low resolution videoframe/video frame of a video thumbnail.

The reconstructed macroblocks are inverse scaled in a step 214 fornormalization. An inverse transform combines the standard basispatterns, weighted by the re-scaled coefficients, to re-create eachblock. A step 216 applies an (n×n) inverse transform on the reduced size(n×n), thus saving the computational expense of doing an inversetransform on a much larger size. A down-sampled and low-resolution blockis finalized in a step 218 and sent in a video thumbnail stream to anoutput 220.

The predictions used in step 212 are formed from the currently decodedframes and previously decoded frames. A step 222 generates Intraprediction reference pixels which are only the bottom and/or right edgesof a block.

A step 224 provides for spatial prediction according to the H.264 videocoding standard. Spatial prediction is used for Inter Prediction.Spatial prediction is also used for Infra block coding. Ordinarily,luminance Intra prediction may be based on a 4×4 for which there arenine prediction modes, or a 16×16 macroblock, for which there are fourprediction modes. For chrominance Intra prediction, there are also fourprediction modes. Lack of availability of top and/or left referencepixels reduce the number of prediction modes employed to produce thevideo thumbnails, and limit the ones employable to those that arecomputationally the simplest and most straightforward.

A step 226 generates few frequency coefficients, (n×n) size, from thespatial predictions using an N×N transform. A step 228 scales theresulting predictions for use in step 212.

Low complexity method embodiments directly decode low-resolution framesfrom compressed high-resolution videos that were encoded using Intrapredictive coding techniques like the H.264 video coding standard. Thesmaller the decoding resolution is, the higher will be the computationand power savings of using the method. Low-frequency coefficients of 2Dtransformed predictions are added to the low-frequency coefficients ofthe transformed residual error. Low-frequency coefficients of thereconstructed data are then inverse transformed taking a smaller size.Further savings are obtained by reconstructing only those referencepixels that will be needed for accurate decoding of subsequent Intrablocks.

FIG. 3 represents a low-resolution video frame/video thumbnail decoder300 with an input 302 to receive a highly compressed video like H.264.The video thumbnail decoder 300 could be built as a field programmablegate array (FPGA) or an application specific integrated circuit (ASIC).An entropy decoder 304 is connected to an inverse zigzag scan 306. Partof the output is directed to an inverse transform unit 308, and theother part to a re-scaling unit 310. Re-scaling is a combination ofprescaling and inverse quantization operations. Prescaling is an scalingoperation which is a part of Modified integer IDCT. An adder 312combines the transformed residual frames to the transformed predictedframes to produce transformed low resolution reconstructed frames. Aninverse scaler 314 normalizes the frames for a smaller size inversetransform unit 316. Down-sampled low resolution blocks 318 are thenproduced for a video thumbnail output 320. The predictions take some ofthe reconstructions to generate reference pixels in a unit 322. Aspatial prediction unit 324 generates prediction data for reconstructinga frame. An N×N transform 326 generates few of the frequencycoefficients for a scaling unit 328.

Decoder 300 decodes single or multiple input streams into one or morecorresponding low-resolution outputs. The H.264 video coding standarduses the 4×4 Integer transform for coding the Inter and intra residualerrors. So in order to decode the low-resolution frames, down-samplingby four in each direction is used to produce one sample for each 4×4block. One 2D DC coefficient is therefore needed per 4×4 block.

In the decoder 300, the non-reference P/B frames, e.g., last P-frame orB-frame just before the IDR frame, is directly sub-sampled forreconstruction. These P-frames and B-frames do not require de-blocking,because at these resolutions the human eye will do the averaging andblocking, making any artifacts not visible.

Intra block down-sampling is by four. H.264 uses the 4×4 integertransform for coding Inter and Intra residual errors. Downsampling byfour in each direction requires generation of one sample per 4×4 block,i.e., one 2D DC coefficient per 4×4 block. The compressed 4×4 block goesthrough entropy decoding, inverse zigzag and inverse quantization. Afterentropy decoding, inverse quantization is done, without a pre-scalingoperation. The 4×4 blocks are down-sampled by four by adding 2D DCcoefficients of the transformed prediction to the 2D DC coefficients ofthe residual error.

Reconstructed DC coefficient=Reconstructed 2D DC coefficient of residualerror+DC Coefficient of 2D Transformed Prediction.

Down-sampled Image pixel=Reconstructed DC coefficient/normalizationfactor

Normalization_factor=4 for H.264

These down-sampled image pixels form an image with a down-samplingfactor of four. The DC coefficients of the transformed prediction arecalculated in an efficient way for various prediction modes and variousblock sizes.

In the Intra block down-sampling by two, only the 2×2 low-frequencycoefficients of a 4×4 block are needed. In the operations involved indown-sampling by two small errors may get introduced because of theapproximations being used. These errors are acceptable because of theway the results are being used in thumbnails. In pseudocode, this couldbe represented by.

For Intra block down-sampling by two, a 2-point 2D inverse transform isused.    $\; {D = \begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}$ x = normalization_factor1 * D X D′ normalization_factor1= 1/normalization_factor*N2*N2, where N2 =   2 for 2×2 transformnormalization factor: This is a normalization factor that depends on  other pre or post operations. X: 2×2 low-frequency coefficients of 2Dtransformed image data. P: 2×2 low-frequency coefficients of 2DTransformed Prediction RE: 2×2 low-frequency coefficients of 2DTransformed Residual Error X = P + RE The Horizontal Predictioncomprises: 2D_Transformed_Prediction_DC of a 4×4 block =4*(J1+J2+J3+J4)/N; 2D_Transformed_Prediction_DC of blocks B1, B2, B3 andB4 of a   16×16 block = 4*(J1+J2+J3+J4) /N; 2D_Transformed_Prediction_DCof blocks B5, B6, B7 and B8 of a   16×16 block = 4*(J5+J6+J7+J8) /N;2D_Transformed_Prediction_DC of blocks B9, B10, B11 and B12   of a 16×16block = 4*(J9+J10+J11+J12) /N; and 2D_Transformed_Prediction_DC ofblocks B13, B14, B15 and   B16 of a 16×16 block = 4*(J13+J14+J15+J16)/N; where, N = 4, according to 4×4 transform used in H.264 standard.

The locations of pixels JI to J16 and K1 to K16 used as reference pixelsare diagrammed in the following Tables.

D0 K1 K2 K3 K4 J1

J2 J3 J4

indicates data missing or illegible when filed

H0 K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14 K15 K16 J1

J2 J3 J4 J5 J6 J7 J8 J9 J10 J11 J12 J13 J14 J15 J16

indicates data missing or illegible when filed

n the horizontal prediction, the 2D transform of horizontal predictionis taken according to H.264. For down-sampling by two, only the 2×2low-frequency coefficients are needed. A first step is to take thehorizontal transform. Since the data is the same along the horizontaldirection, only the DC coefficient will have a non-zero value. The DCcoefficient can be calculated by adding the elements of the rows. Afterthe horizontal transform, only the first column will have non-zeroelements. The vertical transform of only the first column is taken. Onlythe first two coefficients of the first column are calculated. Thecalculation for the 2D transform of horizontal prediction is thereforevery simple and efficient.

Only the first two coefficients of the first column are multiplied withthe corresponding post scaling coefficients 1/4 and

$\frac{1}{2\sqrt{10}}.$

Such post scaling coefficients are in accordance with the H.264 videocoding standard.

Pre-scaling is not needed before taking the 2D inverse transform in unit310.

In pseudocode, this could be represented by,

$P = \begin{bmatrix}\frac{F\; 1}{4} & 0 \\\frac{F\; 2}{2\sqrt{10}} & 0\end{bmatrix}$ X = P + RE x = (1/(normalization_factor*N2*N2)) * D X D′According to H.264 video coding standard, normalization_factor = N4;  where N4 = 4, if the 2D transform of prediction is a 4×4 transform.The Vertical Prediction comprises: 2D_Transformed_Prediction_DC of a 4×4block = 4*(K1+K2+K3+K4) /N; 2D_Transformed_Prediction_DC of blocks B1,B5, B9 and B13   of a 16×16 block = 4*(K1+K2+K3+K4) /N;2D_Transformed_Prediction_DC of blocks B2, B6, B10 and B14 of a   16×16block = 4*(K5+K6+K7+K8) /N; 2D_Transformed_Prediction_DC of blocks B3,B7, B11 and B15 of a   16×16 block = 4*(K9+K10+K11+K12) /N; and2D_Transformed_Prediction_DC of blocks B4, B8, B12 and B16 of a   16×16block = 4*(K13+K14+K15+K16) /N; where, N = 4, according to 4×4 transformused in H.264 standard.

For the vertical prediction, the 2D transform of vertical prediction istaken according to H.264. For down-sampling by two, only the 2×2low-frequency coefficients of vertical prediction are generated.

First, the vertical transform is taken. The data is the same along thevertical direction, so only the DC coefficients will have non-zerovalues. The DC coefficients are easily calculated by adding the elementsof columns. After the vertical transform, only the first row will havenon-zero elements. The horizontal transform of only the first row mustbe taken, and only the first two coefficients of the first row need tobe calculated. The 2D transform of vertical prediction is thuscalculated in an efficient way.

Then, only the first two coefficients of the first row are multipliedwith the corresponding post scaling coefficients ¼ and

$\frac{1}{2\sqrt{10}}.$

These post scaling coefficients comply with the H.264 video codingstandard.

After entropy decoding, inverse quantization is done without apre-scaling operation. In pseudocode, this could be represented by,

$P = \begin{bmatrix}\frac{F\; 1}{4} & \frac{F\; 2}{2\sqrt{10}} \\0 & 0\end{bmatrix}$ X = P + RE x = (1/(normalization_factor*N2*N2)) * D X D′According to the H.264 video coding standard, the normalization_factor =N4; and N4 = 4, when the 2D transform of prediction is a 4×4 transform.The DC Prediction comprises: 2D_Transformed_Prediction_DC of all 4×4blocks = 16 *   Mean_Prediction_Value/N where, N = 4 according to 4×4transform used in H.264 standard.

In the DC Prediction, the 2D transform of the DC prediction is takenaccording to H.264. For down-sampling by two, only the 2×2 low-frequencycoefficients of the DC Prediction need to be generated.

Since the data along the horizontal direction is the same as along thevertical direction, only the 2D DC coefficient will have non-zero valueand we can straight away calculate the 2D DC coefficient by adding theelements of the 4×4 matrix. The 2D transform of DC prediction isefficiently calculated.

Only the 2D DC coefficient is multiplied with the corresponding postscaling coefficients ¼. This post scaling coefficient is in accordancewith H.264 video coding standard. In pseudocode, this could berepresented by,

$P = \begin{bmatrix}\frac{F\; 1}{4} & 0 \\0 & 0\end{bmatrix}$ X = P + RE x = (1/(normalization_factor*N2*N2)) * D X D′According to the H.264 video coding standard, normalization_factor = N4;and N4 = 4, if the 2D transform of prediction is a 4×4 transform. F1 =16 * Mean_Prediction_Value${2{D\_ Transformed}{\_ DC}{\_ Prediction}} = \begin{bmatrix}\frac{F\; 1}{4} & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}$ The Plane Prediction comprises2D_Transformed_Prediction_DC for each 4×4 block = Sum of all plane  predicted values within each 4×4 block/N where, N = 4 according to 4×4transform used in H.264 standard.

The 2D transform of Plane prediction is taken according to H.264. Fordown-sampling by two, just the 2×2 low-frequency coefficients areneeded, so only the 2×2 low-frequency coefficients of Plane Predictionare generated.

The first two coefficients in the vertical transform are calculated foreach column. After the vertical transform, the horizontal transform ofonly the first two rows is taken, only the 2×2 low-frequencycoefficients need to be calculated. Only the first two coefficients ofeach horizontal transform are calculated for a transform. In this way,the 2D transform of plane prediction is calculated in an efficient way.

The first four coefficients of the first row are multiplied with thecorresponding post scaling coefficients ¼,

$\frac{1}{2\sqrt{10}},\frac{1}{2\sqrt{10}},$

¼, in accordance with the H.264 video coding standard.

The 2D inverse transform is taken without the pre-scaling.

In pseudocode, this could be represented by,

$P = \begin{bmatrix}\frac{F\; 1}{4} & \frac{F\; 2}{2\sqrt{10}} \\\frac{F\; 3}{2\sqrt{10}} & \frac{F4}{4}\end{bmatrix}$ X = P + RE x = (1/(normalization_factor*N2*N2)) * D X D′According to H.264 video coding standard, normalization_factor = N4;where N4 = 4, if the 2D transform of prediction is a 4×4 transform. The4×4 Directional Prediction Modes: 2D_Transformed_Prediction_DC for each4×4 block = Sum of all   predicted values within each 4×4 block/N where,N = 4 according to 4×4 transform used in H.264 standard.

The 2D transform of the directional prediction, those other thanhorizontal, vertical and DC, is taken according to H.264. Fordown-sampling by two, only the 2×2 low-frequency coefficients are neededto generate the 2×2 low-frequency coefficients of directionalprediction.

The first two coefficients in the vertical transform are calculated foreach column. After vertical transform, the horizontal transform of onlyfirst two rows are taken because only the 2×2 low-frequency coefficientsare needed. Only the first two coefficients of each horizontal transformare calculated to be able to take the 2×2 transform. The 2D transform ofthe plane prediction is simple and efficient.

Only the first four coefficients of the first row are multiplied withthe corresponding post scaling coefficients ¼,

$\frac{1}{2\sqrt{10}},\frac{1}{2\sqrt{10}},$

¼. These post scaling coefficients are according to the H.264 videocoding standard.

In pseudocode, this could be represented by,

$P = \begin{bmatrix}\frac{F\; 1}{4} & \frac{F\; 2}{2\sqrt{10}} \\\frac{F\; 3}{2\sqrt{10}} & \frac{F4}{4}\end{bmatrix}$ X = P + RE x = (1/(normalization_factor*N2*N2)) * D X D′According to H.264 video coding standard, normalization_factor = N4;  where N4 = 4, if the 2D transform of prediction is a 4×4 transform.

The decoded DC coefficients are combined together to produce thedown-sampled image frame, e.g., with a down-sampling factor of greaterthan four.

A reconstruction of the reference pixels is needed for decoding futureintra blocks. The prediction modes for all of the 4×4 blocks of a 16×16or 8×8 block are present before the encoded residual error. So, whichpixels of a particular 4×4 block that will act as reference pixels isknown. Here, only the reference pixels that are needed will begenerated. If a block to be decoded needs only the top or left referencepixels, then only one or the other are decoded and reconstructed. If itrequires both the top and left reference pixels to be available, thenboth are decoded and reconstructed. Video coding standards allow thePrediction modes of entire frame to be stored together and the residualerror of entire frame is stored together separately, this is generallyknown as “Data Partitioning”. Data. Partitioning allows greater savingsas we can know before hand which particular reference pixels arerequired for decoding future Macroblocks and only the required referencepixels are reconstructed.

For the luminance 16×16 blocks, the H.264 video coding standard usesreference pixels from blocks T1, T2, T3, T4, L1, L2, L3 and L4, tocompress a 16×16 block. The 16×16 block is encoded by splitting it intosixteen 4×4 blocks (B1 to B16), and then each 4×4 block is codedindividually.

TABLE III

The 4×4 blocks B1, B2, B3, B5, B6, B7, B9, B10 and B11, lack thereference data that will be needed by the intra blocks on the right andbottom Macroblock edges. The pixels in the farthest right column, andthe bottom row of these 4×4 blocks are not reconstructed, henceresulting in computation savings. The down-sampling operation for theseblocks requires decoding only the low-frequency coefficients, e.g., the1×1 or 2×2 block located at the top left corner of each 4×4 block. Thereference pixels in the far right column and bottom row of the 4×4 blockneeded for future blocks are generated only in blocks B13, B14, B15,B16, B4, B8, and B12.

If the right/bottom blocks need only the left or top reference pixels,then only those reference pixels are decoded and reconstructed. Ifright/bottom blocks need both top and left reference pixels to beavailable, then both the top and left reference pixels are decoded andreconstructed. The intra blocks for chrominance are 8×8 blocks.

TABLE IV

The 4×4 blocks C1 do not include the reference data that will be neededby the right and bottom 8×8 chrominance blocks. The pixels on the rightmost column and bottom most row of this 4×4 block do not need to begenerated, hence resulting in computation savings. The down-samplingoperation for these blocks requires decoding only the low-frequencycoefficients, e.g., 1×1 or 2×2 block located at the top and left sidecorner of each 4×4 block. The reference pixels needed for future blockson the right column and bottom row of the 4×4 block are generated onlyin blocks C2, C3 and C4.

If the right/bottom blocks require only the left or top referencepixels, then only those top or left reference pixels are decoded andreconstructed. If both top and left reference pixels are required to beavailable, then both are decoded and reconstructed.

In Table-V, E1 represents a luminance 8×8 block. The others, D1, T1, T2,T3, T4, L1 and L2 are 4×4 blocks that include the reference data neededto generate E1.

TABLE V

The down-sampling operation for this 8×8 block requires decoding onlythe low-frequency coefficients, e.g., the 2×1 or 2×2 blocks located inthe top left corner of the 8×8 block. Then the reference pixels neededfor future blocks are generated in the far right column and bottom rowof the 8×8 block.

If the right/bottom blocks require only left or top reference pixels,then only their top or left reference pixels are decoded andreconstructed. If it requires both top and left reference pixels to beavailable, then both top and left reference pixels are decoded andreconstructed.

Reconstructing the Inter frame includes decoding the residual error withentropy decoding, inverse zigzag scan, inverse quantization, and inversetransform. Then, the motion vector is used to select a reference regionfrom the reference frame. A prediction can then be generated from thereference region, and it is added to the decoded residual error. Theprior art reconstructs a full resolution Inter frame, and thendown-samples it using a downsampling filter.

In comparison, the embodiments disclosed herein generate a down-sampledframe directly. A small number of the frequency coefficients ofprediction Obtained from the reference frame are generated. These fewfrequency coefficients are generated using N×N transform, e.g., 8×8.These frequency coefficients are small 2×2) in number, as compared tothe original number (8×8) of frequency coefficients. Frequencycoefficients of the prediction are added to the corresponding frequencycoefficients of the reconstructed residual error. The inverse transform(1×1 or 2×2) is taken of these few frequency coefficients (1×1 or 2×2).The inverse transformed result is then normalized. Normalizeddown-sampled blocks together form a down-sampled image.

If the generated down-sampled frame is to be used as a down-sampledreference frame, then the reference DC coefficient for the next frame isgenerated from the down-sampled frame by weighted average of fourreference DC coefficients. The DC coefficient of the reference block ofthe next frame is within the region covered by these four reference DCcoefficients. Using the down-sampled frames as reference frames willintroduce drift errors, a mismatch between the decoded frames on encoderand decoder sides. Such drift errors can either be tolerated, or theirvisual impact can be mitigated by post-processing techniques.

Table-VI illustrates an example of a slice group with maximum errorrobustness and the embodiments yield maximum computation savings. H.264has specified a few explicit Slice Groups, and it allows custom slices.Macroblocks can be chosen in random, and then assigned to two slicegroups. However, users must provide maps of the Macro block in the SliceGroup to the Macro block in the frame.

TABLE V1 SGI SGII SGI SGII SGI SGII SGI SGII SGII SGI SGII SGI SGII SGISGII SGI SGI SGII SGI SGII SGI SGII SGI SGII SGII SGI SGII SGI SGII SGISGII SGI SGI SGII SGI SGII SGI SGII SGI SGII SGII SGI SGII SGI SGII SGISGII SGI SGI SGII SGI SGII SGI SGII SGI SGII SGII SGI SGII SGI SGII SGISGII SGI

Table-VII represents another example of Slice Groups and Slices. Oneslice group is split in to two slices (S1 & S2) and another has beensplit in to two slices (S4, S5 & S6).

Video thumbnails are generally preferred over static image thumbnails. Avideo thumbnail will provide better insights into what the parent videoincludes. DVDs, video archives, and video directories are betterrepresented by video thumbnails, as compared to a static thumbnailimage. Video thumbnail, low-resolution content can be extracted ondemand from the user, for example, action scenes, etc.

Low-resolution content can be extracted by decoding a low-resolutionbitstream, or by extracting and displaying low-resolution content fromthe high-resolution compressed video, such as a few low-resolution I, Por B frames. The locations of key frames can be stored in asupplementary information file. This supplementary information file canbe obtained by parsing video stream on-the-fly, or before hand.Considering the computation complexity and delays, in most cases havingthe information a priori would be best. Frame identifiers are gleanedfrom the bitstream or from storing similar information during a playbacksession. Information from the file is used to extract low-resolutioncontent from these key frames.

FIG. 4 is a set top box embodiment 400. Several video feeds 401-403 areinput to a tuner 410 with a program selector switch 412. An H.264decoder decompresses the selected video bitstream for viewing by a useron a main viewer 420. Such a user is able to see which program intereststhem by low-resolution extraction units 421-423 that drive videothumbnails 431-433. The low-resolution extraction units 421-423 embodysoftware and hardware similar to method 200 (FIG. 2) and decoder 300(FIG. 3). The video thumbnails 431-433 can be presented in tandem onmain viewer 420, each as a small picture-in-picture (PIP), or on asecondary video display.

FIG. 5 represents a method embodiment 500 for sub-sampling of Intrablocks. A step 502 is used to generate the frequency coefficients of theprediction needed. These frequency coefficients will be small in number,compared to the original number of frequency coefficients that wouldordinarily be generated. A step 504 generates the correspondingfrequency coefficients of the residual error. In a step 506, only thosepixels of a particular block are reconstructed that will be used asreference pixels for decoding future blocks. In a step 508, thefrequency coefficients of the prediction needed are added to thecorresponding frequency coefficients of the residual error. In a step510, the inverse transform of the frequency coefficients needed aretaken, and will be very much reduced in number compared to the startingnumber of frequency coefficients. In a step 512, the resultingdown-sampled image is normalized. In a step 514, if the down-sampledframe generated is to be used as a reference frame, then the referenceDC coefficients for a next frame are generated in a step 516 by aweighted average of four reference DC coefficients. The DC coefficientof the reference block of the next frame is within the region covered bythe four reference DC coefficients. Using down-sampled Intra frames asreference frames for Intra frame reconstruction will introduce drifterrors which can either be tolerated, or their visual impact can bereduced in a post-processing step 518.

FIG. 6 represents a method 600 for sub-sampling of the inter blocks. Ina step 602, the reference block selected in the reference frame ispointed to by the integer portion of the motion vector. The fractionalpart of the motion vector is disregarded, and frequency coefficientsneeded are generated in a step 604 from the selected reference block.The number of these frequency coefficients are small (1×1, 2×2) innumber, compared to the original number. Having to generate a predictionfor the fractional part of the motion vector adds a considerablecomputational load, so dropping the fractional part in the embodimentsmakes this sub-sampling relatively efficient. In a step 606, thecorresponding frequency coefficients of the residual error aregenerated, where steps of generation are entropy decoding, inversezigzag scan and inverse quantization. In a step 608, the frequencycoefficients of the prediction needed are added to the correspondingfrequency coefficients of the reconstructed residual error. The inversetransform of the needed few frequency coefficients is taken, in a step610. These few frequency coefficients are small (1×1, 2×2) in number, ascompared to the original number of frequency coefficients. A step 612normalizes the resulting down-sampled block. The normalized down-sampledblocks together form a down-sampled image. In a step 614, if thedown-sampled frame generated is to be used as a reference frame, then areference DC coefficient for a next frame is generated from it in a step616 using a weighted average of four reference DC coefficients. The DCcoefficients of the reference block of the next frame will be within theregion covered by these four reference DC coefficients. Using suchdown-sampled frames as reference frames will introduce drift errors.These drift errors can either be tolerated, or the visual impact ofdrift errors can be reduced by a post-processing step 618.

FIG. 7 represents a video thumbnail method embodiment 700. Videothumbnails are time variant visual interfaces of smaller display size,as compared to the resolution of a main display device. This timevariant visual interface is an interfacial representation of theunderlying stored data. A low resolution video bitstream is generated ina step 702. Low-resolution video bitstreams can be used as supplementaryvideo files for the corresponding high-resolution video bitstreams. In astep 704, supplementary information files can be used to index thelocation and other properties of key frames in video bitstreams, theseother properties are frame display time, frame type etc. A step 706associates and stores supplementary video and information files alongwith their corresponding video bitstreams. The supplementary files andhigh-resolution video bitstream are usually combined in a singlecontainer file format. Both the supplementary files and high-resolutionvideo bitstream get encapsulated within a single container file format.A step 708 extracts the selected low-resolution frames from ahigh-resolution stored compressed video file. Frames to be decoded areeither chosen on-the-fly from the high-resolution coded bitstream, orare indexed in a high-resolution coded bitstream with the help ofsupplementary information file. Frames can also be decoded from acorresponding low-resolution encoded video. When a user opens a videoarchive in a step 710, the computer system decodes the selected frames,and displays only low-resolution frames. The time variant content(frames) of one or more video files is simultaneously visuallycommunicated to the user in a step 712.

In some situations, less than all of the top and left reference pixelsmay be available. A prediction mode may have to be chosen from a limitednumber of prediction modes, increasing the probability of choosing asimple prediction mode. If the prediction mode is simple, greatercomputation savings are realized.

The embodiments are beneficial when all the I-frames in a video sequenceare being decoded, and when error resiliency features such as slices andslice groups are being used. These are important for wireless channelsand networks that are prone to packet losses.

Decoding and reconstructing only a few samples results in computationsavings. The I and B Frames are especially difficult to decode, so theembodiments are helpful in applications that require display ofdown-sampled frames. For example, in the Multimedia entertainment, DataArchiving and Medical imaging industries, Set Top Boxes, entertainment,handheld devices, etc., and also in security applications in shoppingmalls, banks, airports etc., that require monitoring multiple inputsobtained from various cameras installed at different locations thatrequire scaling before they can be shown. A video thumbnail is moreappealing as compared to a static image thumbnail. DVDs/videoarchiving/video directory can have a video thumbnail as compared to astatic thumbnail. A user may want to see multiple videos that are beingstreamed over the Internet, and want to choose between multiplebroadcast programs, hence requiring simultaneous display of videos.

Some applications require encoding of complete video using only Intraframes. A master copy of the original video is generally coded as an allIntra frame encoded video sequence. The H.264 standard specifies allIntra Profile's that are targeted towards “New profiles for professionalapplications”.

The embodiments described here have generally related to video framesrepresented in YUV/YCbCr color format, but alternative embodimentsinclude RGB color and other formats. The most recent Intra PredictionModes of H.264 video coding standard have been described, but futuretypes of Intra prediction modes could no doubt be used with thesetechniques and methods.

Although specific embodiments have been described and illustrated, theembodiments are not to be limited to the specific forms or arrangementsof parts as described and illustrated herein. The embodiments arelimited only by the claims.

What is claimed is: 1-7. (canceled)
 8. A system comprising: adown-sampler operable to produce a first low-resolution frame from aselected high-resolution compressed frame, the first low-resolutionframe corresponding to a first DC coefficient and a plurality offrequency coefficients; and a frame reconstruction unit operable toproduce a second low-resolution frame according to the plurality offrequency coefficients of the first low-resolution frame, the secondlow-resolution frame having a second DC coefficient, wherein the framereconstruction unit is operable to produce a third low-resolution frameaccording to a residual error between the first low-resolution frame andthe second low-resolution frame, a third DC coefficient for the thirdlow-resolution frame being a weighted average of the first DCcoefficient and the second DC coefficient.
 9. The system of claim 8,wherein the selected high-resolution compressed frame is a bi-predictiveframe (B-frame) of a group of pictures (GOP) and is located immediatelybefore an instantaneous decoding refresh (IDR frame).
 10. The system ofclaim 8, wherein the selected high-resolution compressed frame is apredictive frame (P-frame) of a group of pictures (GOP) and is locatedimmediately before an instantaneous decoding refresh (IDR frame). 11.The system of claim 8, wherein the frame reconstruction unit is operableto add low frequency coefficients of a 2D transformed prediction to lowfrequency coefficients of the residual error to produce low frequencycoefficients of the third low-resolution frame.
 12. The system of claim8, wherein the frame reconstruction unit is operable to produce onlythose reference pixels required for an accurate decoding of Intrablocks.
 13. A system comprising: a down-sampler operable to produce afirst low-resolution frame from a selected high-resolution compressedframe, wherein the selected high-resolution compressed frame is an intraframe (I-frame) of a group of pictures (GOP); and a frame reconstructionunit operable to: generate frequency coefficients of a prediction,wherein the number of frequency coefficients of the prediction is nomore than one fourth of the frequency coefficients in the I-frame of theGOP; generate frequency coefficients of a residual error; reconstructthose pixels of a particular block that will be used as reference pixelsfor decoding future blocks; take the inverse transform of a sum of thefrequency coefficients of the prediction and the frequency coefficientsof the residual error; and generate a reference DC coefficient for anext frame as a weighted average of four reference DC coefficients. 14.The system of claim 13, wherein the down-sampler is operable tosub-sample I-frames in a predictive coding video bitstream by: using aninteger portion of a motion vector to point to a reference blockselected in a reference frame; generating a first set of frequencycoefficients from the selected reference block, wherein the number offrequency coefficients in the first set is no more than one fourth ofthe frequency coefficients in the selected reference block; generating asecond set of frequency coefficients for a residual error; adding thefirst set of frequency coefficients to the second set of frequencycoefficients; and taking an inverse transform of the sum of thefrequency coefficients.
 15. The system of claim 13, wherein thelow-resolution frame is used as a reference frame.
 16. A method forextracting a low-resolution video, comprising: inputting a videocomprising a plurality of high-resolution frames, at least one of theplurality of high-resolution frames being a reference frame, theplurality of high-resolution frames being encoded with a predictivecoding technique; down-sampling the reference frame to produce a firstlow-resolution frame with a first DC coefficient; generating a pluralityof frequency coefficients corresponding to the first low-resolutionframe; generating a second low-resolution frame according to theplurality of frequency coefficients and the first low-resolution frame,the second low-resolution frame having a second DC coefficient;generating a residual error between the first low-resolution frame andthe second low-resolution frame; generating a third low-resolution frameaccording to the plurality of frequency coefficients and the residualerror; and determining a third DC coefficient for the thirdlow-resolution frame according to a weighted average of at least thefirst DC coefficient and the second DC coefficient.
 17. The method ofclaim 16, wherein the plurality of high-resolution frames comprises acompressed predictive frame that immediately precedes an InstantaneousDecoding Refresh (IDR) frame.
 18. The method of claim 16, the methodcomprising: adding a low frequency coefficient of a two dimensionaltransformed prediction to the residual error; and taking an inversetransform.
 19. The method of claim 16, the method comprising:reconstructing a plurality of reference pixels required for an decodingan Intra block.
 20. The method of claim 16, the method comprising:extracting a low resolution Intra frame from a high resolutioncompressed Intra frame by: generating frequency coefficients of aprediction needed, wherein the number of frequency coefficients are afraction of the number of frequency coefficients in the high resolutioncompressed Intra frame being sampled; generating corresponding frequencycoefficients of the residual error; reconstructing pixels of aparticular block that will be used as reference pixels for decodingfuture blocks; adding the frequency coefficients of the predictionneeded to corresponding frequency coefficients of the residual error;taking the inverse transform of the frequency coefficients; normalizinga resulting down-sampled image; and if a down-sampled frame generated isto be used as a reference frame, then any reference DC coefficients fora next frame are generated by a weighted average of four reference DCcoefficients, so the DC coefficient of the reference block of the nextframe are within a region covered by the four reference DC coefficients;wherein using down-sampled Intra frames as reference frames for Intraframe reconstruction introduces drift errors which are accepted.
 21. Themethod of claim 16, the method comprising sub-sampling an Inter block ina predictive coding video bitstream by: using an integer portion of amotion vector to point to a reference block selected in a referenceframe; disregarding the fractional part of the motion vector; generatingfrequency coefficients needed from a selected reference block, wherein,the number of frequency coefficients is no than one fourth of thefrequency coefficients in an Intra frame; dropping a fractional part insub-sampling; generating corresponding frequency coefficients of aresidual error; adding frequency coefficients of a prediction needed tocorresponding frequency coefficients of a reconstructed residual error;taking an inverse transform of one or more frequency coefficients; andnormalizing a resulting down-sampled block to form a down-sampled image.22. The method of claim 16, the method comprising: generating areference DC coefficient for a next frame using a weighted average offour reference DC coefficients, wherein the next frame is to be used asa reference frame.